Let us first clarify the notions. The data set has one data point in it which is either (heart attack) or (no heart attack). Being a function of means being a function of that value (), so indeed is a function of , and it so happens in this problem that the value is fixed at . The probability is also a function of (which happens to be the same as according to what we are conditioning on).

Since is fixed, this leaves as a function of only.

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